A proximity switch of high frequency oscillation type has a detection coil in its detection head and an oscillation circuit is formed therein using the detection coil as an oscillation coil, so as to detect an object from the decline in the oscillation output due to the decline in the inductance of the oscillation coil.
One form of oscillation circuit of current feedback type which may be used in a proximity switch is given in FIG. 4. In this drawing, an oscillation circuit is formed by connecting a capacitor C in parallel to a detection coil L which is provided in the front part of the proximity switch. This LC resonance circuit receives electric current from a constant current source 2 by way of a power source 3 and an end thereof is connected to a transistor 4 for current amplification. The emitter of the transistor 4 is grounded by way of a variable resistor 5 which determines the collector current, and a current mirror circuit CM1 consisting of a pair of transistor 6 and 7 is connected to the collector of the transistor 4.
The current produced from the LC resonance circuit is current amplified by the transistor 4 and a current of the same amplitude as the collector current is current fed back by the current mirror circuit CM1 to the LC resonance circuit by way of the transistor 7. On account of this positive current feedback, the LC resonance circuit starts an oscillation at its resonance frequency.
According to this oscillation circuit, the speed of termination of the oscillation may be varied by adjusting the shape and winding of the detection coil L and changing other circuit constants. However, when an object approaches the detection coil and increases the conductance of the detection coil, the oscillation is terminated sooner or later. Therefore, the response speed of a proximity switch may be considered as the total time of the start and the termination of oscillation. Generally speaking, oscillation of an oscillation circuit is extremely slow in rise (start) speed but is relatively fast in termination speed. The time tau required for oscillation to start, grow and reach a level which enables the detection of an object may be given as follows: ##EQU1## where Vo : Output reversion level
Vs : Amplitude level at the beginning of oscillation PA1 C : Capacitance of the resonance capacitor PA1 g : Variation of conductance from the beginning of oscillation.
Generally, the amplitude level at the termination of oscillation is a noise level which is, for instance, in the order of mV's. As the detection distance is increased, the conductance variation decreases and the response speed decreases, causing the problem that the detection of a fast moving object becomes impossible. To avoid such a problem, a proposal has been made to provide a initialization signal generator for giving an initialization signal to the oscillator circuit (Japanese Patent Laying-Open Publication No. 58-1327), but the initialization signal must be matched to the oscillation frequency and the circuitry tends to be complicated.
Furthermore, in an environment where a strong AC magnetic current is present as in a resistance welder which involves an electric current of more than ten thousand A, the ferrite core of the detector coil is saturated and, due to the increase in the loss in the detection coil, the oscillation is interrupted. Therefore, in such an environment, a conventional proximity switch of high frequency oscillation type may become unusable.